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Pushing the thermal limits of nanoscale SQUIDs

Superconducting Quantum Interference Devices (SQUIDs) are incredibly sensitive magnetic flux sensors which consist of a small superconducting loop containing one or two weakened regions of superconductivity known as Josephson elements.

Recently there has been a global trend to develop ever smaller nanoscale SQUIDs with the ultimate goal to reach a sensitivity (which scales with the loop size) sufficient to detect the flip of one or more electron spins in close proximity. This goal represents a grand challenge in many areas of applied physics including spintronics, biomagnetics, nano-electromagnetics, and especially spin-based quantum information processing. The Josephson elements are typically realized by patterning nanoscale constrictions (‘nanobridges’) in superconducting thin films using either e-beam or focused ion-beam lithography.

However, up to now it has often been very difficult to realize the full potential of these nanoscale SQUIDs at the ultralow (millikelvin) temperatures required for most quantum measurements and applications. This is because SQUIDs are usually operated with their Josephson elements current-biased in the normal state which generates excess heat by the Joule effect. Removing this heat at ultralow temperatures is extremely challenging because thin film interfaces and superconducting materials are both very poor thermal conductors at such low temperatures.

In the new work, researchers from UCL and the UK National Physical Laboratory (NPL) have succeeded in overcoming this difficulty by introducing a new type of nanoscale SQUID based on optimised proximity effect bilayers. This consists of a noble metal (Au) layer as a thermal shunt which is integrated with a variable-thickness superconducting (Ti) layer. The noble metal layer helps control the superconducting transition temperature and the degree of heating in the device via the so-called superconducting proximity effect. Dr Edward Romans, who led the research at UCL, said “the wider temperature range of operation of our new devices will give us tremendous flexibility to exploit them for many quantum measurements in the future.”

This work has been published in Journal of Applied Physics (A. Blois et al, J. Appl. Phys. 114, 233907 (2013)).

Figure: SEM micrographs from different perspectives (false coloured for clarity) of a nanoSQUID fabricated by e-beam lithography in the LCN. The device is made from a Ti(100nm)/Au(20nm) bilayer (blue), and was used to make ultrasensitive magnetic measurements down to 60 mK. The nanobridges (Josephson elements) are only 50 nm wide.